Control rod and a control rod blade for a boiling water reactor

ABSTRACT

The present invention relates to a control rod blade for a boiling water reactor. The control rod blade ( 2 ) comprises a free edge portion with a recess ( 7 ) which comprises a plurality of outlets, arranged in a row, for channels ( 3 ), which are arranged to receive an absorber material ( 10 ) and a cover element ( 4 ) arranged to be attached along at least a section of the edge portion. The cover element ( 4 ) comprises a cover portion ( 12 ) arranged to seal the opening of the recess ( 7 ) and a support portion ( 13 ) arranged to, in a mounted state, abut a bottom surface in the recess ( 7 ) and to allow the formation of at least a passage ( 16 ) between the outlets of the channel ( 3 ) in the recess ( 7 ).

THE BACKGROUND OF THE INVENTION AND PRIOR ART

[0001] The present invention relates to a control rod and a control rodblade for a boiling water reactor, wherein the control rod bladecomprises a free edge portion with a recess which comprises a pluralityof outlets, arranged in a row, for channels, which are arranged toreceive an absorber material and a cover element arranged to be attachedalong at least a section of the edge portion, which cover elementcomprises a cover portion arranged to seal the opening of the recess anda support portion arranged to, in a mounted state, abut a wall surfacein the recess and to allow the formation of at least one passage betweenthe outlets of the channels in the recess.

[0002] Usually, control rods for boiling water reactors (BWR) comprisesa central part and four control rod blades protruding from the centralpart, which each is provided at a right angle in relation to theadjacent control rod blades. Usually, conventional control rod bladesare manufactured of a sheet of a steel material. The control rod bladesare provided with free edge portion with a longitudinally milled recessa. A plurality of channels are drilled in the recess in a row above eachother. The channels have a diameter, which is less than the thickness ofthe metal sheet such that a satisfactory wall thickness is obtained. Thechannels extend from the free edge portion of the control rod bladetowards the central part of the control rods. A suitable absorbermaterial is arranged in the channels. Thereafter, the absorber materialis enclosed by rolling the legs of the recess together. Thereafter, theend surfaces of the legs are welded together with a longitudinal weldjoint such a hermetic and pressure resistant sealing of the recess isobtained. A relatively deep recess must here be provided at the edgeportion of the control rod blade such that the legs will obtain a lengthsuch that they can be rolled together. In a welded state, the free edgeportion of the control rod blade here gets a rounded shape andconsequently, the absorber material has to be arranged at a notcompletely negligible distance from the end surface of the control rodblade. In order to increase the reactivity effect of the control rodblade, it would be desirable if the absorber material could be arrangedcloser to the end surface.

[0003] By a profylax published in Research Disclosure 33925/92 a controlrod blade is shown, which is provided with a concave recess at a freeedge portion. In order to seal the recess, a cover element, which has asubstantially corresponding shape, is here applied in the recess. Thecover element comprises a cover portion, which seals the opening of therecess, and a support portion, which comprises a convex contact surfacearranged to abut a corresponding concave wall surface of the recess. Thecover element is fixed by means of two longitudinal weld joints to thelegs of the recess. The support portion comprises a chamfered portionsuch that a passage is formed between the chamfered wall surface of thesupport portion and the concave wall surface of the recess, whichpassage connects the outlets of the channels in the space to each other.The channels here contain a powdered absorber material. The passageallows a flow of the fission gases, which are created during operation,between the channels such that a pressure equalization is obtained. Thecover element has a shape, which allows the arrangement of two furtherpassages. These further passages are arranged substantially immediatelyinside the weld joints. Passages allow distribution of a protective gasto the root side of the weld joints during the welding operation. Thecover element has in this case a relatively complicated construction atthe same time as a concave-shaped recess results in that the outlets ofthe channels, at least locally, are located at a relatively largedistance from the end surface of the control rod blade. The possibilityto arrange the absorber material near the end surface of the control rodblade is thereby reduced.

SUMMARY OF THE INVENTION

[0004] The object of the present invention is to provide a control rodblade having a free edge portion which has a relatively simpleconstruction such that the control rod blade may be manufactured to alow cost at the same time as the construction of the edge portionresults in that the control rod blade obtains a high reactivity effectduring operation.

[0005] The above-mentioned objects are achieved with the control rodblade of the initially mentioned kind, which is characterized in thatthe support portion has a width which is less than the width of therecess and a contact surface which is arranged to abut a bottom surfacein the recess. Since the support portion has a width which is less thanthe width of the recess, at least on one side of the support portion,but preferably on both sides of the support portion, a passage isobtained between a side wall surface of the support portion and the sidewall surface in the recess. Such a passage allows for a flow of fissiongases during operation such that the pressure between the channels,which lead to the recess, is equalized. Such a cover element may begiven a relatively uncomplicated shape and may by that means bemanufactured to a low cost. By such co-lateral passages, the recess canbe made shallow and the outlets of the channels are thus obtained at arelatively short distance from the end surface of the control rod blade.Thereby, the absorber material can also be arranged at a relativelyshort distance from the end surface of the control rod blade such thatthe control rod blade achieves a high reactivity effect duringoperation.

[0006] According to a preferred embodiment of the invention, the supportportion has a width, which is less than the width of the outlets of thechannels. Thereby, the support portion only partly covers the outlets ofthe channels and the created fission gases are allowed to flow past thesupport portion and through the passage to adjacent channels.Advantageously, the support portion has a continuos extension along thewhole length of the cover element. An attachment element, whichcomprises a support portion with a continues extension, has asubstantially constant cross-section profile along its extension, whichallows for a simple manufacturing to a low cost. Such an attachmentelement also provides a simple mounting since no particular adaptationneed to be done of the support portion in relation to the outlets of thechannels for obtaining a satisfactory function. Advantageously, thecover element has a substantially T-shaped cross-section profile.Advantageously, a T-shaped cover element comprises an upper coverportion with a substantially rectangular cross-section profile with awidth, which substantially covers the opening of the recess and athickness, which allows for a pressure resistant sealing of the recess.Advantageously, the support portion also has a substantially rectangularcross-section profile with a perpendicularly downwardly extendingextension in the recess from a central part of the cover portion. Such aT-shaped cover element is simple to manufacture.

[0007] According to another preferred embodiment of the invention, thesupport portion has a substantially plane contact surface arranged toabut a corresponding plane bottom surface in the recess. With a planecontact surface, no local cavities of the space are obtained in thecontact area. Such local cavities increase the distance from the outletsof the channels to the end surface of the control rod blade and hencethe distance of the absorber material to the end surface. The bottomsurface of the recess may be formed as a groove in the areas, whichextend between the outlets of the channels. Preferably, said groove hashere a width, which corresponds to the width of the support portion.Thereby, the support portion of the cover element obtains in a simpleway a correct and stable positioning in the recess during the montage.Consequently, the cover portion of the cover element coverssubstantially an opening of the recess in said position. However, withsuch a positioning, narrow slits between the edges of the cover elementand the legs of the recess may be obtained. Such slits facilitates awelding of the edges of the cover element to the inner surfaces of thelegs when the slits allow for distribution of a protective gas to thepassage such that the protective gas may act against the underside ofthe formed weld.

[0008] According to another preferred embodiment of the invention, thecover element is attached to the edge portion of the control rod bladeby two longitudinal weld joints. Advantageously, such weld joints may beachieved with a TIG-burner. Consequently, a T-shaped cover elementprovides a passage on each side of the support portion. Hence, thepassages allow in addition to the pressure equalization during operationalso a distribution of an inert gas to the underside of the weld duringthe welding operation. By such a supply of an inert gas, it is securedthat the weld joints are formed with a high quality.

[0009] According to another preferred embodiment of the presentinvention, said absorber material consists of solid absorber bodies.Advantageously, solid absorber bodies are rod shaped and has a circularcross-section profile. Solid absorber bodies have the advantage inrelation to powdered absorber material that the absorber material heredoes not risk to whirl up during the welding operation and to be mixedin the weld joint. Advantageously, said solid absorber bodies compriseboron carbide. Boron carbide is the most frequent absorber material andmay be pressed to solid absorber rods by means of a HIP (Hot IsostaticPressing) process. Alternatively, absorber rods of hafnium may be used.One or several absorber rods may be arranged in each of the channels.When absorber rods of boron carbide absorb neutron radiation, theyexpand. In order to allow for an expansion of the absorber rod in anaxial direction in the channel, a spring may be arranged furthest backin the channel. Thereby, the outer end of the absorber rod can beprovided relatively near the cover element without the absorber rodrisking to deform the cover element during an axial expansion.

SHORT DESCRIPTION OF THE DRAWINGS

[0010] In the following, an example of a preferred embodiment of theinvention is described with reference to the attached drawings, inwhich:

[0011]FIG. 1 shows a schematic perspective view of a control rod for aboiling water reactor,

[0012]FIG. 2 shows an edge portion of a control rod blade and a coverelement according to the present invention,

[0013]FIG. 3 shows a sectional view along the line A-A in FIG. 2 and

[0014]FIG. 4 shows a sectional view of an edge portion of a control rodblade having a mounted cover element.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

[0015]FIG. 1 shows a control rod 1 for controlling of the neutron fluxin a boiling water reactor (BWR). The control rod 1 comprises fourcontrol rod blades 2, which are directed in a right angle in relation tothe adjacent control rod blades 2. The control rod blades 2 comprise aplurality above each other, in a row, drilled channels 3, which extendbetween a free edge portion of the control rod blades 2 and a center,where the control rod blades 2 meet. The control rod blades 2 aremanufactured of a sheet material, which preferably is of steel and has athickness of 7-8 mm. The channels 3, which are drilled in the controlrod blades 2 may here have a diameter of 5-6 mm.

[0016]FIG. 2 shows both an edge portion of a control rod blade 2 beforeit is enclosed and a cover element 4 for sealing the edge portion.Consequently, the control rod blade comprises a sheet material the freeend surface of which is provided with a first U-shaped track 5 by meansof a milling operation. The first track 5 has a constant width duringits extension along substantially the whole free end surface of thesheet material. Thereafter, a second narrower U-shaped track 6 has beenmilled centrally in the first U-shaped track 5. The second track 6 alsohas a constant width during its extension along substantially the wholefree end surface of the sheet material. The first 5 and second 6U-shaped tracks together form a recess 7 in the free edge portion of thecontrol rod blade 2. The recess 7 is restricted laterally by legs 8having a suitable wall thickness. After or before the manufacturing ofthe recess 7, a large number of straight channels 3 with a circularcross section have been drilled in the control rod blade 2.Consequently, the channels 3 extend between the free edge portion of thecontrol rod blade 2 and a center, where the control rod blades 2 meet.The channels 3 have a diameter, which is less than the width of thefirst U-shaped track 5 but which exceeds the width of the secondU-shaped track 6. The centrally arranged second U-shaped track 6 thusobtains only an extension in the surface areas 9 which are formedbetween the outlets of the channels 3 in the recess 7. FIG. 2 showsoutlets of six drilled channels 3. The three lowest located channels 3have here been provided with absorber material in the form of solidabsorber rods 10, which, with advantage, are manufactured of boroncarbide. The absorber rods 10 have a smaller diameter than the channels3 such they are allowed to expand radially when they are subjected toradiation during operation. A spring, which is not shown in the figures,is arranged furthest back in the channels 3 such that the absorber rods10 with a yielding pressure are held in the direction towards theoutlets of the channels. The absorber rods 10 are allowed to expandaxially in the channels 3 against the action of the spring. The absorberrods 10 comprise a chamfering 11 such that its thickness decreases atthe outer end. Since the end surface of the absorber rods 10 is alsosubjected to radiation, the absorber rods are subjected to the greatestradiation dose at the outer end. By such a chamfering 11, one securesthat the absorber rod 10 in this area does not obtain a radial extensionsuch the absorber rod 10 will abut the surrounding channel wall.

[0017]FIG. 2 thus show also the cover element 4, which is arranged toseal the recess 7 such that the control rod blade 2 obtains an externaleven end surface. The cover element 4 comprises a length, whichcorresponds to the length of the recess 7, and has a cover portion 12,which has a width corresponding to the width of the first U-shaped track5. The upper surface of the cover portion 12 is arranged tosubstantially form the end surface of the control rod blade in a mountedstate. A support portion 13 of the cover element 4 is marked in FIG. 2with broken lines. The support portion 13 has a continuos extensionalong the whole length of the cover element 4 and has a width whichsubstantially corresponds to the width of the second track 6.

[0018]FIG. 3 shows a sectional view along the line A-A in FIG. 2. Here,the first U-shaped track 5 and the design and the depth of the secondU-shaped track 6 are more evident than in FIG. 2. The cover element 4has a substantially T-shaped cross-section profile. The cover portion 12of the cover element 4 comprises a substantially upper rectangular crosssection area with a wall thickness dimensioned for enclosing fissiongases with a specific pressure and a width which substantiallycorresponds to the width of the opening of the recess 7. The supportportion 13 of the cover element 4 comprises a lower substantiallyrectangular cross section area, which has a width substantiallycorresponding to the width of the second U-shaped track 6. The supportportion 13 has a height such that a plane lower contact surface 14 abutsa corresponding plane bottom surface 15 of the second U-shaped track 6when the upper surface of the cover portion 12 is on a level with theend surface of the legs 8. An absorber rod 10 having a chamfering 11 atthe end is arranged in the channel 3.

[0019]FIG. 4 shows a cross section view of an edge portion of a controlrod blade 2 with a mounted cover element 4. Since the support portion 13has a width which is less than the width of the recess 7, passages 16are obtained on both sides of the support portion 13, which extend alongthe recess 7. The passages 16 connect the outlets of the channels 3 toeach other and thereby allow for a flow of fission gases between thechannels 3 during operation. The overpressure, which is created by thefission gases during operation, may thereby be distributed substantiallyuniformly between all the channels 3, which lead to the space 7 of thecontrol rod blade 2.

[0020] The second U-shaped track 6 thus has a width, which substantiallycorresponds to the width of the support portion 13. Thereby, the coverelement 4 obtains, during the montage, a stable positioning in therecess 7 such narrow uniform slits are formed on both sides of the coverelement 4 between the edges of the cover portion 12 and the legs 8 ofthe recess 7. Such slits allow for the distribution of a protective gasto the passages 16 during a welding operation of the edges of the coverportion 12 to the inner surfaces of the legs 8. The cover element 4 ishere attached in the recess 7 by means of two longitudinal weld joints17. Advantageously, such weld joints 17 may be provided with aTIG-burner. Consequently, a T-shaped cover element 4 provides a passage16 on both sides of the support portion 13. The passages 16 allow, inaddition to the above mentioned pressure equalization function duringoperation, hence also for a distribution of an inert gas to root side ofthe weld joint during the welding operation. By such a supply of aninert gas, it is secured that the weld joints 17 obtain a high quality.

[0021] The present invention is not in any way restricted to theembodiment shown in the drawings but may be freely modified within thescope of the claims. The recess does not necessarily need to consist oftwo U-shaped tracks but may have a substantially arbitrary butfunctional shape.

1. A control rod blade for a boiling water reactor, wherein the controlrod blade comprises a free edge portion with a recess which comprises aplurality of outlets, arranged in a row, for channels, which arearranged to receive an absorber material and a cover element arranged tobe attached along at least a section of the edge portion, which coverelement comprises a cover portion arranged to seal the opening of therecess and a support portion arranged to, in a mounted state, abut awall surface in the recess and to allow the formation of at least onepassage between the outlets of the channels in the recess, wherein thesupport portion has a width which is less than the width of the recessand a contact surface which is arranged to abut a bottom surface in therecess.
 2. The control rod blade according to claim 1, wherein thesupport portion has a width which is less than the width of the outletof the channels.
 3. The control rod blade according to claim 2, whereinthe support portion has a continuous extension along the whole length ofthe cover element.
 4. The control rod blade according to claim 1,wherein the cover element has a substantially T-shaped cross-sectionprofile.
 5. The control rod blade according to claim 1, characterized inthat the support portion has a substantially plane contact surface whichis arranged to abut a corresponding plane bottom surface in the recess.6. The control rod blade according to claim 1, wherein the bottomsurface is shaped as a groove in the areas, which extend between theoutlets of the channel.
 7. The control rod blade according to claim 6,wherein the groove has a width which substantially corresponds to thewidth of the support portion.
 8. The control rod blade according toclaim 1, wherein the cover element is attached to the edge portion ofthe control rod blade by two longitudinal weld joints.
 9. The controlrod blade according to claim 1, wherein said absorber material consistsof solid absorber bodies.
 10. The control rod blade according to claim9, wherein said solid absorber bodies comprise boron carbide.
 11. Thecontrol rod for a boiling water reactor comprising at least a controlrod blade according to claim 1.